Conventional methods of butyl rubber manufacture employ temperatures of −100° C. to −90° C., methyl chloride as a diluent, and a Lewis acid co-initiator such as aluminum chloride. Under these conditions, production of high molecular weight (typically greater than 200,000) butyl rubber, which is a co-polymer of isobutylene (i.e., isobutene) and 0.5–2.5 weight percent isoprene, occurs at acceptable rates and where water in combination with the Lewis acid is effective for protic initiation of polymerization. Methyl chloride is useful as it is both a polar solvent that enhances propagation rates and a poor solvent for butyl rubber so that the process is a suspension polymerization at these low temperatures. About 500 million pounds of butyl rubber was produced in the United States in 1991.
Legislation passed in the United States allows the use of methyl chloride in existing plants. However, expansion and/or construction of new plants will require the use of alternative solvents that are not chlorinated. Hence, there is a need to develop initiators that will be effective in the absence of a halogenated solvent in producing high molecular weight butyl rubber at commercially acceptable rates. Ideally, the process is a suspension polymerization so as to facilitate heat and mass transfer. Solution polymerization in liquid or diluted monomer and a supported catalyst are also possibilities.
A variety of Lewis acidic main group and transition metal initiators or co-initiators of isobutylene polymerization have been reported to provide poly(isobutene) (PIB) or co-polymers of isobutylene and isoprene in the absence of chlorinated solvents or with a minimum amount of chlorinated solvents being present. None of these compositions actually provide butyl rubber of sufficiently high molecular weight at acceptable rates in the absence of chlorinated solvents. Hence, there is a continuing need to develop more effective initiator compositions.
Chelating diboranes have been investigated as co-catalysts in combination with metallocene dialkyls in ethylene polymerization. Generically, these compounds can be formulated as R′2B—R—BR′2 where R is a covalent linking atom or group, R′ is an organic substituent and R is of a length that allows the two boron (B) atoms to cooperate in the binding of suitable anions or donors. More specifically, R and R′ are both perfluorinated alkyl groups, most preferably perfluoroaryl substituents so that the boron atoms are highly Lewis acidic, but hydrolytically stable and soluble in non-polar solvents. In addition, triphenylmethyl diborates ([Ph3C][R(BR′3)2(μ−X)] with X═F, N3, OMe, OC6F5, and R and R′ as above) in which the diborate counter-anion has a group X bridging the two boron atoms were also investigated and some of these compositions were more effective as co-catalysts in ethylene polymerization than mononuclear versions such as [Ph3C][B(C6F5)4], which is in commercial use.